Microlithographic projection exposure apparatuses are used to transfer structures arranged on a mask to a light-sensitive layer such as a photoresist, for example. The light-sensitive layer is usually situated on a wafer or some other substrate. The projection exposure apparatus typically includes a light source, an illumination system, which conditions projection light generated by the light source and directs it onto the mask, and a lens, which images the mask illuminated by the projection light onto the light-sensitive layer.
The shorter the wavelength of the projection light, the smaller the structures can be produced on the light-sensitive layer with the aid of the projection exposure apparatus. The most recent generation of projection exposure apparatuses uses projection light having a centre wavelength of approximately 13.5 nm, which is thus in the extreme ultraviolet spectral range (EUV). Such apparatuses are often referred to as EUV projection exposure apparatuses.
However, there are no optical materials which have a sufficiently high transmissivity for such short wavelengths. Therefore, in EUV projection exposure apparatuses the lens elements and other refractive optical elements that are customary at longer wavelengths are replaced by mirrors, and the mask therefore contains a pattern of reflective structures. The mirrors include a mirror substrate having a reflection region, which is formed on a surface of the mirror substrate and in which the mirror substrate bears a reflective coating. The mirrors are often fixed in support frames that are connected to a rigid frame structure of the lens or of the illumination system via actuators. The entire, inherently rigid assembly including a mirror and, if appropriate, supporting frames fixed thereto, or other components, is referred to hereinafter as mirror element.
The mirror elements of the illumination system direct the projection light onto the mask; the mirror elements of the lens image the region illuminated on the mask onto the light-sensitive layer.
In order to accomplish this with the desired accuracy, the reflection regions of the mirror elements have to be aligned precisely with one another in all six degrees of freedom. Electrically actuable actuators are usually used for positioning and aligning the mirror elements.
EUV projection exposure apparatuses having a large numerical aperture involve mirror elements having a large diameter. Such mirror elements are costly to produce and, owing to their high inherent weight, make it more difficult to implement mounting and actuation with little deformation. Since the mirror element is not an ideal rigid body, the shape of the mirror substrate can vary, e.g. in the long term on account of material degradations and in the short term as a result of the influence of forces and moments that act on the mirror substrate during actuation.
The mirror substrate for mirror elements of an EUV projection exposure apparatus usually consists of a material which has a very low or even vanishing coefficient of thermal expansion at the operating temperature. Such materials may be e.g. glass ceramics such as Zerodur® or titanium silicate glasses such as ULE®. Nevertheless, deformations that are caused thermally can occur in the mirror substrates, the deformations arising as a result of a temperature gradient within the substrate on account of absorption of part of the projection light.
Since the surface accuracy of the reflection region has to be maintained down to a few nanometers during the operation of an EUV projection exposure apparatus, extreme demands with regard to mechanical precision have to be placed on the mounting and actuation of the mirror elements as well. The mounting has to be configured in such a way that undesired and parasitic forces or moments are reduced as much as possible. In order to achieve a high mechanical precision, all factors of potential mechanical disturbances such as, for example, vibrations, air pressure fluctuations or temperature fluctuations and gravitational influences and also material properties have to be taken into account.
In EUV projection exposure apparatuses, each mirror element is usually actuated via three actuators which act on the mirror element and which simultaneously constitute mounting elements. The forces exerted by the actuators can be subdivided into a portion that is responsible only for the movement of the mirror element during positioning and alignment, and a portion that compensates for the weight force of the mirror element. The effective lines of the XY- and Z-forces for actuation and of the weight compensation force meet at the force application point of the respective actuators. Such mirror elements are known from US 2015/0055112 A1, for example. The mounting elements here act on a circumferential surface of the mirror substrate.
EP 1 806 610 A1 discloses a mirror element of an EUV projection exposure apparatus in which the mirror is enclosed in a support frame and the arrangement of the three force application points on the support frame is chosen such that the weight force of the mirror element is distributed uniformly along the three force application points. In the case of force application points distributed uniformly over the circumference of the mirror and in the case of a non-rotationally symmetrical mirror substrate, this is achieved via compensation weights on the support frame. In this case, the effective line of the weight force intersects the optically effective reflection region.
On account of specific boundary conditions such as, for example, construction space conflicts, desired dynamic or thermal properties, this type of mounting can prove to be difficult to implement and restrict the range of possible optical designs. Specifically, the three actuators or other mirror elements arranged on the circumference of the mirror element occupy considerable construction space if the mirror element has a large diameter. Moreover, the mirror element can deform in a manner that is difficult to foresee in the event of temperature changes owing to the fixed clamping at the three mounting elements in the vicinity of its reflection region.
WO 2011/029467 A1 discloses using a mount for the mirror substrate, which mount is carried by actuators and supports the mirror substrate only at a single bearing surface. In order to minimize parasitic forces and moments and resultant deformations of the mirror substrate, the mirror element is fixed at a mounting element of the mount in such a way that the effective line of the gravitational force passes through the geometric centre of the bearing surface at which the mirror substrate touches the mounting element. The mirror element is thus supported by the mount directly below its centre of gravity.